Engine emission control systems may utilize various exhaust sensors. One example sensor may be a particulate matter (PM) sensor, which indicates particulate matter mass and/or concentration in the exhaust gas. In one example, the PM sensor may operate by accumulating particulate matter over time and providing an indication of the degree of accumulation as a measure of exhaust particulate matter levels. The PM sensor may be located upstream and/or downstream of a diesel particulate filter, and may be used to sense particulate matter loading on the particulate filter and diagnose operation of the particulate filter.
PM sensors may encounter issues with non-uniform deposition of soot on the sensor due to a bias in flow distribution across the surface of the sensor. Further, PM sensors may be prone to contamination from an impingement of water droplets and/or larger particulates present in the exhaust gases. If larger particulates adhere to electrodes of the sensor, the PM sensor may no longer be able to reliably measure the PM quantity passed through the DPF. If the condensed water adheres to the electrodes of the sensor element, for example, the accuracy of the PM sensor may be compromised. In addition, the condensed water adhering to the sensor element may cause the sensor element to get cracked due to thermal stress. This can result in warranty issues.
Various approaches have been developed to reduce the non-uniform deposition of soot on PM sensors. One example PM sensor is shown by Zhang et al. in US 2015/0355067 A1. Therein, the PM sensor includes a cylindrical protection tube having perforations, and a sensor element is positioned inside the tube facing towards the perforations. The PM sensor is fixed to an exhaust passage downstream of a particulate filter in such a way that the perforations are located on a downstream surface of the protection tube, facing towards a tail end of the exhaust passage. In such a configuration, exhaust gas flowing through the exhaust passage may experience pressure variations along the exterior of the protection tube. Because of the higher static pressure at the downstream surface relative to the side surfaces, exhaust gas may be drawn towards the perforations on the downstream surface of the protection tube, and the exhaust may enter the PM sensor via the perforations in a direction opposite to the direction of exhaust flow inside the exhaust passage. Because of their larger momentum, the larger particulates and the water droplets may not be able to undergo a reversal in direction of flow and enter the PM sensor.
However, the inventors herein have recognized potential issues with such systems. As one example, the amount of exhaust gas flowing into the PM sensor may be limited by the size, and shape of the perforations. In addition, since the exhaust gas has to undergo a complete reversal in flow direction to enter the PM sensor, the flow rate of exhaust gas entering the sensor may be reduced. This may, in turn, lead to reduced sensitivity of the PM sensor.
In one example, the issues described above may be addressed by a particulate matter (PM) assembly comprising a bent tube having a first closed end and a second outwardly flared end, a plurality of perforations formed proximate to the first end, and a sensor element positioned facing towards the plurality of perforations, the sensor element located upstream of the second end. In this way, the second end of the bent tube may form a venturi that serves to block larger particulates from entering the assembly, and additionally serves to increase exhaust flow into the sensor assembly. As a result, the sensitivity of the sensor may be increased.
As one example, an exhaust PM sensor assembly may be configured with sensor electrodes and may be positioned downstream of a particulate filter in an exhaust pipe. The PM sensor assembly may include a bent, protection tube that forms an L-shape. A first end of the bent tube may be closed, and coupled to the exhaust pipe. A sensor element may be positioned inside the assembly in front of a plurality of perforations; the sensor element, and the plurality of perforations are both located closer to the first end. A second end of the bent tube may be an open, outwardly flared end and may be located downstream of the first end (and the sensor element) and positioned within the exhaust pipe. The bent tube may have a uniform cross-section over the entire length, except at the second end. At the second end, the tube may include an outwardly angled portion that has increasing cross-section all the way to the tip of the bent tube, thus forming a venturi at the second end of the bent tube. As such, at the venturi, exhaust flows from the tip of the tube where the cross-section is higher, towards the first end of the bent tube with the smaller cross-section. As a result, the exhaust encounters a constriction within which the exhaust flow velocity is increased. The exhaust is then directed from the venturi that is located at the second end through the plurality of perforations towards the sensor element that is located proximate to the first end of the protection tube. Specifically, the exhaust is directed towards electrodes of the sensor element. When a controller applies a voltage across the electrodes of the sensor element, the particulates in the exhaust may be captured across the electrodes. Thus, an increased exhaust flow into the venturi may translate into an increased PM deposition across the electrodes of the sensor element. Thereby, the PM sensor may give an accurate measure of the exhaust particulates in the exhaust passage upstream of the particulate filter. In this way, the PM sensor may be used to diagnose leaks in the particulate filter in a reliable manner. When a soot load on the electrodes reaches a threshold soot load, the controller may apply a voltage to heat a heating element coupled to the sensor element to burn off the particulates deposited on the electrodes, thus regenerating the PM sensor.
In this way, by creating a venturi-like structure inside the PM sensor assembly, the exhaust flow inside the PM assembly may be increased, with a corresponding increase in sensor sensitivity. In addition, as the exhaust is streamed from the downstream side of the bent tube, the amount of larger particulates and/or water droplets impinging on the sensor element may be reduced. Specifically, due to their larger momentum, water droplets and/or larger particulates may flow past the venturi without redirecting their flow direction to enter the venturi. Therefore, the sensor element may be protected from impingement of water droplets and larger particulates. Overall, the functioning of the PM sensor assembly may be improved and PM sensor output may be more reliable.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.